The present invention is directed generally to ophthalmic lenses and, in particular, to ophthalmic lenses having progressively varying focal power.
Eyeglasses or spectacles are indispensable for many persons. Even for people who do not need them now, eyeglasses may one day be required since the majority of people's eyes lose their ability to provide adequate vision. Lenses used in eyeglasses have several general requirements such as that they provide a clearer view to the wearer when in use, that they be thin and light, that they have a good appearance and that they not be easily damaged.
Heretofore, various materials for lenses were developed and marketed. At present, however, there are no lenses which satisfy all of the above requirements. This is because the refractive index of such materials must be large in order to make the lenses thin. However, the materials having a large refractive index have a small Abbe's number in general, and chromatic aberration generated at the periphery of the lens makes the outline of an object appear blurred and colored. This chromatic abberation is not so important for a single vision lens directed to remedy general myopia or hyperopia since ordinarily when eyeglasses are used, a wearer hardly uses the periphery of the lens, but, instead, uses portions of the lens proximate the center thereof.
Progressive multifocal lenses are intended to provide a person with presbyopia, i.e. a person whose ability to control his crystalline eye lenses has weakened, with an ophthalmic lens which compensates for such loss. Such a progressive multifocal ophthalmic lens has a far vision viewing zone or far zone in the upper portion of the lens, a near vision viewing zone or near zone in the lower portion of the lens and an intermediate vision viewing zone or interemediate zone in the lens portion intermediate the far and near zones of the upper and lower portions, respectively, of the lens. Each of the zones are smoothly adjacent to one another and are generally constructed from an aspherical refractive surface.
In the case of progressive multifocal lenses, chromatic aberration is a significant problem because of the three viewing zones in the lens. The near zone is about 15 to 25 mm below the center of the lens and chromatic abberation is a problem here. Because of this problem, conventional materials having a high refractive index are generally not used for progressive multifocal lenses.
Accordingly, progressive multifocal ophthalmic lenses which overcome the problems noted above including the problem of chromatic abberation, which improve the distribution of astigmatism and which are thinner and lighter, are desired.